The present invention pertains to a process for the preparation of a solution containing glucose and fructose by the conversion of a glucose-containing solution in the presence of a catalyst having glucose isomerase activity which is prepared based on a SiO.sub.2 carrier.
The enzymatic conversion of glucose into a glucose-fructose mixture has recently taken on a greater significance. The mixture is sold most often in the form of a syrup, called isomerose, and serves as a replacement, above all, in the food and beverage industry, for crystal sugar which worldwide is becoming less abundant and more expensive, i.e., sugar such as cane sugar, beet sugar and saccharose which are produced from sugar cane or sugar beets. As a source for the glucose which is required for the production of the glucose-fructose mixture, there serve naturally available starches, e.g., corn starch, or potato starch, which are converted into glucose by acidic and/or enzymatic hydrolysis.
The enzymatic conversion of glucose into fructose by means of glucose isomerase is well known in the art. The isomerization can be effected by treating an aqueous glucose solution with a glucose isomerase, whereby depending on the type of glucose isomerase additional agents which promote isomerization (e.g. cofactors such as cobalt and/or magnesium ions) may be added to the solution. The solution is subjected to the effect of the glucose isomerase until the desired degree of isomerization is achieved, then optionally any added ingredients may be removed from the solution and the solution may be concentrated to form a syrup.
In the past, it was conventional to operate in a batch-wise manner and to use either isolated glucose isomerase or glucose isomerase left in the natural cellular association of the micro-organism which produces the glucose isomerase. In view of the fact that glucose isomerase utilized in this form either cannot at all or can only at considerable expense be recovered and reutilized, more recently it is becoming more common to utilize glucose isomerase which is made water-insoluble by fixation in the cellular association and which is mechanically stabilized by appropriate additives. Furthermore, glucose isomerase can be adsorptively or covalently bound on inorganic or organic carriers and can thereby be made water-insoluble. A glucose isomerase which in this manner is fixed and stabilized or is bonded to a carrier can be reutilized several times. Thus it becomes possible to operate in a continuous manner for example, in a process whereby the carrier-bonded glucose isomerase is filled into a reactor and the glucose solution (i.e., the substrate) is permitted to flow through the reactor.
The carrier-bonded glucose isomerase which is necessary for such a continuous process (hereinafter referred to as a catalyst or a supported catalyst having glucose isomerase activity) forms a part of the state of the art. For example, see U.S. Pat. No. 4,230,803, which describes in particular a catalyst having glucose isomerase activity which is prepared based on a SiO.sub.2 carrier.